The demand for wireless services has progressively increased since cellular telephony was first introduced. A recurring problem is how to allocate scarce radio resources to an ever increasing number of wireless users. Various wireless protocols have been devised to share the available bandwidth with a plurality of users, including TDMA, GSM and CDMA.
Another way to increase the number or users is to reuse the wireless spectrum over a geographic area by subdividing the service area into cells, each serviced with a base station. Cells are then subdivided into sectors, typically 3 per cell, with the base station including a directional antenna per sector.
In general, it is desired to have as few base stations as possible. This is because base stations are expensive and require extensive effort in obtaining planning permission. In some areas, suitable base station sites may not be available. In order to have as few base stations as possible, each base station ideally has as large a capacity as possible in order to service as large a number of wireless terminals as possible. Several key parameters that determine the capacity of a CDMA digital cellular system are: processing gain, ratio of energy per bit to noise power, voice activity factor, frequency reuse efficiency and the number of sectors in the cell-site antenna system.
AABS (Adaptive Antenna Beam Selection) is a method used in CDMA cellular Base Stations to improve traffic capacity in “hot spot” sectors without requiring additional carriers (i.e. more spectrum) at the hot spot. This spectrally efficient technique replaces the standard sector antenna beam pattern with a multiplicity, typically three, of beams per sector. In other words, to further increase capacity, sectors have been subdivided into beams.
These new beams have higher directivity on both the forward and reverse links. This higher directivity reduces the forward interference seen by a terminal and reduces the received interference level at the base station's receiver. Consequently, the RF power required to support a typical call in the forward ink is lower than that required for a conventional antenna beam. This results in a significantly greater number of AABS calls being supportable with a base station's limited transmitter power than is possible with a conventional sector beam.
In a similar manner to the forward link situation, the reverse link AABS beams are more directive than a conventional sector beam. As a result, the terminal's RF power required to support a typical call in an AABS sector will be lower than for a conventional sector call. This will also help prolong the terminal's battery life.
An example of such an AABS system is described in U.S. patent application Ser. No. 10/698,395, filed Nov. 3, 2003 and U.S. application Ser. No. 09/733,059 which was filed on Dec. 11, 2000, which are hereby incorporated in reference in their entirety.
In the past, wireless networks primarily provided telephony services (i.e., voice calls). Today, wireless networks are evolving to provide a variety of data services, including interactive, real-time, and delay-sensitive applications, such as VoIP, video conferencing, mobile gaming, mobile music, and high-speed file transfers. As wireless spectrum is limited, there is a need for a more efficient allocation of radio resources to support different types of users whose bandwidth requirements can vary substantially. Increasingly, wireless users want access to a diverse set of data and multimedia applications with different bandwidth demands, as well as to real-time applications (such as gaming and video) where minimum performance guarantees are required in terms of bandwidth, delay, and bit error rate.
One approach to satisfy these requirements is the CDMA 2000 1xEV-DO standard, which was introduced to handle significantly higher data rates on the downlink (for web browsing, for example) and to efficiently implement a packet data service, which was constrained because voice and data in traditional CDMA systems were carried over the same radio frequency (RF) carrier. Such a system is described in the article CDMA2000 1xEV-DO: An Easy Upgrade Path to Mobile Broadband Services by Vish Nandlall, published in the July 2005 edition of the Nortel Technical Journal, which is hereby incorporated by reference. 1xEV-DO has been defined by the Third Generation Partnership Project 2 (3GPP2), a collaboration of several standards bodies from around the world that is developing technical specifications and a framework for third-generation CDMA wireless networks. Specifically, 3GPP2 has defined a data-only version of CDMA called CDMA2000 High Rate Packet Data (HRPD)—more commonly referred to as 1xEV-DO. The “1x” prefix stems from its use of 1 times the 1.2288 mega chips per second (Mcp) spreading rate of a standard IS-95 CDMA channel. “EV” emphasizes that it is an EVolutionary technology that builds and improves on CDMA 2000 technology. The “DO” (data optimized) suffix indicates that 1xEV-DO is designed to efficiently transfer data. The 3GPP2 technical specification for 1xEV-DO is C.S0024-A v. 1.0, and has been published as a North American standard by the Telecommunications Industry Association (TIA) as IS-856, which is hereby incorporated by reference.
One advantage of the 1xEV-DO systems is they provide a high bandwidth (“fat pipe”) data channel, which can be shared by a plurality of users who are serviced by the same sector of a base station. At a high level, the 1 xEV-DO divides a CDMA channel into timeslots, and assigns user data to each timeslot. Accordingly, 1xEV-DO is combines CDMA with time division multiplexing (TDM) to increase capacity.
However, the IS-856 standard does not provide any mechanism for providing additional capacity in “hot spots” by dividing a 1xEV-DO sector into beams. One of the advantages of using beam antennas is it increases capacity. However this increase in capacity is achieved conventionally by sectorization, i.e., the division of an area into smaller areas in order to increase capacity by reusing resources in smaller areas. A known problem with increased sectorization is interference. It becomes increasing more difficult to “plan” pilot PN's or “assign” PN offsets that do not degrade the network performance because the re-use distance gets smaller and smaller as the pilot density increases due to uncontrolled sectorization. This effect is known in the industry as “pilot Pollution” and has a number of deleterious impacts on a network including dropped call rate increases and access failure rate increases. Consequently sectorization density above a certain limit in a network should be avoided if possible. We avoid this problem by re-using PN offsets within a sector, even if the sector is divided into beams. In other words, each beam within a sector uses the set of PN offsets allocated to the sector, thus not increasing number of PN offsets per BTS (or per unit area) due to this re-use.
It is, therefore, desirable to provide a base station which can maximize throughput for maximum number of data users in a sector.